#1, no ideas on this one. #2, sounds like you've got an intermittent hardware issue. This would account for the "turn on" issues and can also fool XP into thinking it's been moved to a new computer. This requires re-activation. Call the 800 number, re-activation only takes a few minutes. I would then have the hardware checked out and hopefully find a permanent solution to the problem.
#3 the limit/full switch keeps the lens in non-macro mode in the limit position for faster focusing. The full setting allows the lens to extend to it's maximum (closest focus) length. This answer is based on my experience my Tamron lenses with the full/limit feature. HTH Don
